Acyloxysilanes have been prepared by the reaction of an alkylchlorosilane with a carboxylic acid. The reaction is reversible and is brought to completion by removing the by-product HCl either by addition of a proton acceptor or by distilling it off at high temperature. The reaction mechanism can be represented as follows: EQU R.sub.n SiCl.sub.4-n +(4-n)CH.sub.3 COOH.fwdarw.R.sub.n Si(OCOCH.sub.3).sub.4-n +(4-n)HCl (I)
where R is an alkyl radical and n is an integer from 0 to 3.
The reaction scheme of (I) is the subject of various methods for improving the yield of the desired end product, including the addition of an iron complexing agent, (U.S. Pat. No. 3,974,198); conducting the reaction under reflux conditions using pentane as a solvent (U.S. Pat. No. 4,028,391); conducting the reaction in the vapor phase in a counter-current column (U.S. Pat. Nos. 4,329,484 and 4,332,956); and refluxing the reactants in carbon tetrachloride as a solvent (Japanese patent document 80,154,983 (1980)).
These reaction schemes have the disadvantages of requiring either (1) additional reactants to neutralize or otherwise remove the HCl, or (2) additional utilities and equipment to conduct the reaction and to remove the HCl from the reactor in order to drive the reaction to completion.
Alkylacyloxsilanes have also been obtained by the reaction of a carboxylic acid anhydride with an alkylchlorosilane: EQU R.sub.n SiCl.sub.4-n +(4-n)(CH.sub.3 CO).sub.2 O.fwdarw.R.sub.n Si(OCOCH.sub.3).sub.4-n +(4-n)CH.sub.3 COCl (II)
where R is an alkyl radical and n is an integer from 0 to 3.
This reaction (II) is brought to completion by distilling off the acetyl chloride formed. Acetyl chloride is corrosive, toxic, a strong irritant and presents a significant fire risk. It must be handled carefully and disposed of properly. A modification of this process is disclosed in European patent EP 509,213.
The technical literature also discloses other processes relating to specific acyloxysilanes. For example, Hengge, E. and Starz, E. (Montash. Chem. 102(3) 741-6 1971; CA 75, 88066) reported the preparation of triacetoxysilane (11.8% yield) by treating trichlorosilane with sodium acetate in pentane at room temperature. Kohma S. and Matsumoto, S. (Kagaku To Kogyo (Osaka) 48(8) 308-10 1974; CA 82 57798) improved the yield (48-64%) in the preparation of alkylacetoxysilanes by refluxing the chlorosilanes with magnesium acetate in benzene. The use of a metal salt of the acid in these processes results in production of a metal chloride, either NaCl or MgCl.sub.2, which requires additional washing steps for product recovery.
It is therefore a principal object of the present invention to provide an improved one-step process for the production of acyloxysilanes by reacting a halosilane with other relatively inexpensive reactants.
It is another object of the present invention to provide an improved process for producing acyloxysilanes at moderate temperatures in the range from about -5.degree. C. to about 50.degree..
Another object of this invention is to provide a process for the production of acyloxysilanes that does not require a catalyst.
A further object of this invention is to provide a process for the production of acyloxysilanes that does not require a high-temperature distillation process and that can be carried out in conventional reactors.
It is also an important object of the invention to provide a process in which the acyloxysilanes are obtained in yields exceeding at least 75%, and up to 95%.
It is an additional object of the present invention to provide an improved process for the production of acyloxysilanes in which the by-products are non-hazardous and can readily be disposed of in accordance with existing environmental regulations.
Still another object of the present invention is to provide a process for producing acyloxysilanes in which the solvent can be recycled, and where the acyloxysilane remaining in the reaction vessel after removal of the solvent is sufficiently pure so that it can be used without further purification.